Safety valve specification

[Drexl]

Hi,

I'm reviewing some safety devices for boiler and piping system according to european standards (EN 754-7..).

The set pressure of the safety valves is the design pressure of the system, and the pressure can exceed this by 10% when the safety device is operating.

The standard specifies that the pressure drop in the inlet pipe to the safety valve should be <3% of the set pressure. Also pipe sizes before/after the safety valve should not be smaller than for the safety valve.

The outlet piping should be designed so that the velocity is not supersonic and so that the 10% rule is not exceeded.

Now in the data sheet from the manufacturer the built-up back-pressure is said to be 0 bar and superimposed pressure is atmospheric.

Is it correct not to consider any built-up back pressure as long as this is below some limit (as with the 3% inlet rule) or is the safety valve specified wrong. There is of course some piping after the valve.

Is there some other rule i should know about? For example regarding maximum lengths of piping or similar.

 

[sheiko]

Hello,

I believe it is not correct. If the built-up backpressure is substantially low, then i would put an arbitrary value like 10% of the set pressure, so that a conventionnal safety valve can be selected.

BTW, in Europe, design pessure = maximum allowable pressure (PS).

"We don't believe things because they are true, things are true because we believe them."

 

[vpl]

The set pressure of the safety valves is the design pressure of the system, and the pressure can exceed this by 10% when the safety device is operating.

Out of curiosity, how do you manage to get the pressure to go up once the valves open? Most places, the pressure would drop.

Patricia Lougheed

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[Drexl]

vpl,
Well by supplying a bigger massflow than the safety device lets out. Say we have a spring loaded valve opening at 10 bar, surely it will not let out the rated flow at 10.1 bar so the pressure keeps rising. When the pressure reaches 11 bar, the valve should let out the rated flow, otherwise you have chosen too small a valve, the pressure keeps rising and you have a safety issue.

 

[Drexl]

Ok i found a quote regarding outlet piping "The EN ISO 4126 standard states that the pressure drop should be maintained below 10% of the set pressure", I don't have the standard but i assume this is correct.

But the question still remains if 0 pressure drop is mentioned in the supplier data sheet, has he assumed that the outlet piping is designed according to the standard, thus having a pressure drop of 0-10% of the set pressure. And the valve will work as supposed in this case. I think i convinced myself, but please say if you disagree.

The delivery time for safety valves can be quite long, so the piping design is not final when the valves are ordered. Allowing the backpressure to vary between 0 and 10% of the set pressure without separate notice sounds logical.

 

[iainuts]

Hi Drexl. The advice regarding 10% back pressure is typically meant for a conventional, spring loaded relief valve with no bellows or other sealing means on the outlet to balance the valve. For such a valve, the poppet is exposed to inlet pressure on one side and discharge pressure on the other.

First, let's assume the valve will be shut when discharge pressure increases. In this case, the set pressure increases 1 for 1. If the outlet pressure increases 1 bar, the pressure it will lift at will increase by 1 bar. That's simply due to the force on the poppet being the summation of forces:
1. Inlet pressure times area lifting poppet.
2. Spring load forcing poppet shut.
3. Discharge pressure times area forcing poppet shut.

Area in all cases is the sealed area of the seat.

When the valve lifts, the pressure area changes. There's a "huddling chamber" which exposes a slightly larger area to the inlet pressure. That's what snaps the valve open. Because the area suddenly increases, the force tending to open the valve suddenly increases, accelerating the poppet to the full open position. In this case, the forces get a bit complicated because of the dynamic pressure drop across the orifice, but in general, these types of spring loaded relief valves are made to remain open when back pressure builds up to 10% or even greater. This allows for some pressure drop to atmosphere when flow is forced through the discharge piping.

I've seen some valves function normally at much higher pressure on the outlet. It depends on the design. But if back pressure exceeds some pressure (10% is generally a minimum), the additional force could close the valve, leading to valve chatter. Therefore, the standards typically use 10% as the amount of back pressure during flow that a valve can operate normally under. If back pressure exceeds 10% during flow, the valve may be forced closed, resulting in chatter.

Note the 3% on the inlet addresses a very similar issue. If there's a large drop in pressure during flow, there won't be enough flow/pressure to hold the valve open, and it will shut. At that point, pressure builds up very quickly, reopening the valve. Again, the valve has a tendency to chatter.

Bottom line - the piping system must limit pressure drop on the inlet to 3% of set pressure and pressure drop on the outlet to 10% of set pressure.

 

[GenB]

Note that sect VIII and I have different blowoff settings set by the National Board and the ASME Code Sections.
check your UV for sect VII-1 and U for Sect I,

 

[rneill]

You might also want to look at the following API Standards which provide an excellent overview on all issues related to sizing, selection and installation of PSV's

API 520 Part 1, Sizing and Selection of Pressure Relieving Devices in Refineries
API 520 Part 2, Installation of Pressure Relieving Devices in Refineries

These standards address in detail the inlet pressure drop, the permissible accumulated backpressure on conventional spring operated valves (typically 10%) as well as balanced bellows valves (somewhere between 30 and 50%) and the issue of how much the valve may exceed the set pressure under it's rated flowing conditions (dependent on the design code in use).

In addition to the issues talked about, we typically want the set pressure of the relief valve to be at least some margin above the maximum operating pressure in order to prevent problems with inadvertent lifting or leaking, and to deal with valve blowdown. For a conventional valve, typical practice would be to maintain a 10% margin whereas for pilot operated valves a margin of 3 to 5% may be okay.

 

[davefitz]

Drexyl,

I have not read the referenced EN754-7, but your summary sounds a bit off what would be practical for a boilr.

In the US a boiler is normally a drum type boiler, while in europe most"steam generators" are once thru units without a steam drum. I make that distinction, because in the US a boiler with steam drum cannot be overpressured by more than 7% of the master stamping pressure. So, you might first determine what your jurisdiction permits for the type of "boiler" you have.

Next, the outlet piping almost always must be of larger size than the relief valve inlet piping, and that size must be specifically determined in each case based on the length of the discharge pipe and number of elbows, etc, so as to ensure the backpressure at the relief valve outlet elbow is less than ( TBD- above posts suggest 10% of rated pressure).

There are other finer points, such as allowance for error in pressure setting, and interaction with other pressure relief systems ( eg, bypass systems)that may operate in parrellel with the relief valve.

Finally , in some parts of europe, some bypass control valves are rated to serve as releif valves, but I would not recommend using such a design on a drum type boiler.